Glucocorticoid-induced osteoporosis is a serious bone disorder caused by direct and indirect actions of glucocorticoids on bond formation and bone resorption. Glucocorticoids have important effects on cells of the osteoblastic lineage; they decrease the number of bone forming cells and regulate genes expressed by the osteoblast. The effects of glucocorticoids on genes regulating various components of the bone matrix appear central to their inhibitory actions on bone formation. Cortisol inhibits type I collagen synthesis at the transcriptional and post-transcriptional level in cultures of osteoblast enriched cells by post-transcriptional mechanisms, and collagenase 3 plays an important role in bone resorption and in bone and cartilage matrix degradation. The Principal Investigator has discovered that cortisol increases the binding of cytosolic proteins from osteoblast cell extracts to specific regions of the collagenase 3 RNA. The goal is to extend these observations and determine RNA sequences and cytosolic proteins responsible for the effect of cortisol on collagenase 3 mRNA stabilization. Specific aims are: 1) The identification of RNA sequences responsible for the stabilization of collagenase 3 mRNA by cortisol, by characterizing complexes formed by specific collagenase 3 RNA probes and cytosol extracts from osteoblast cells in an RNA gel mobility shift assay; 2) The isolation and characterization of cytosolic proteins responsible for the stabilization of collagenase 3 mRNA by cortisol, using conventional biochemical techniques, an expression osteoblast cDNA library probed with specific RNA probes or novel three hybrid system; and 3) The study of the expression and regulation of genes for RNA binding proteins and the definition of the biological effect of RNA binding proteins in osteoblasts by their over expression in osteoblastic cell lines and examination of changes in the osteoblastic phenotype. This investigation should provide important information on the mechanism involved in the actions of glucocorticoids in bone and increase our understanding of post-transcriptional control in osteoblasts.